1. Field of the Invention
The present invention relates to a field effect transistor (“FET”) having a gold layer, a microfluidic device including the FET and a method of detecting an analyte having a thiol group using the FET and the microfluidic device.
2. Description of the Related Art
A class of sensors for detecting biomolecules with electrical signals includes transistor-based biosensors having a structure including a transistor. These transistor-based biosensors are produced by applying the techniques of semiconductor processes, and are advantageous in terms of rapid conversion of electrical signals, easy integration of microelectromechanical systems (“MEMS”) with integrated circuits and the like. Thus, considerable research has been conducted on these transistor-based biosensors in recent years.
The first patented invention to detect biological reactions using a field effect transistor (“FET”) is disclosed in U.S. Pat. No. 4,238,757. The invention of the '757 patent relates to a biosensor for detecting an antigen-antibody reaction by measuring a current generated as a result of a change in a semiconductor inversion layer induced by a change in the surface charge concentration, and the biosensor is targeted to proteins among various biomolecules. U.S. Pat. No. 4,777,019 discloses a device for adsorbing a biological monomer on a surface of the gate of a FET, and measuring the extent of hybridization of a biological monomer with a complementary monomer using the FET.
U.S. Pat. No. 5,846,708 describes a method of determining whether a bound biomolecule is hybridized, by measuring the light absorption of the bound biomolecule using a charged coupled device (“CCD”). U.S. Pat. Nos. 5,466,348 and 6,203,981 describe methods of using a thin film transistor (“TFT”) to enhance the signal-to-noise ratio by integrating the TFT with a circuit.
When a FET is used as a biosensor as described above, the FET is advantageously less expensive and detects biomolecules in less time than conventional methods. Furthermore, the FET used as the biosensor is easy to integrate into an integrated circuit (“IC”)/MEMS process.
The structure of a conventional FET is schematically illustrated in FIG. 1A. Referring to FIG. 1A, the FET includes a substrate 11 which is either doped with n-type or p-type impurities, a source 12a and a drain 12b which are formed apart from each other on two edges of the substrate 11 and a gate 13 which is formed on the substrate 11 to be in contact with the source 12a and the drain 12b. The source 12a and the drain 12b are doped to include a polarity opposite to that of the substrate 11. A channel is generally formed between the source 12a and the drain 12b. The gate 13 generally includes an oxide layer 14, a polysilicon layer 15 and a gate electrode layer 16, and probe biomolecules 18 are attached to the gate electrode layer 16. A reference electrode 17 is formed apart from the gate electrode. The probe biomolecules 18 bind with predetermined target biomolecules (not shown) by hydrogen bonding or the like, and this binding is electrically measured to determine the degree of binding between the probe biomolecules 18 and the target biomolecules.
FIG. 1B is a diagram schematically illustrating the process of immobilizing probe biomolecules 18 on the surface of the gate electrode layer 16, and allowing the probe biomolecules 18 to bind with the target biomolecules. Referring to FIG. 1B, an intensity of the current flowing through the channel may vary according to whether or not the probe biomolecules 18 are immobilized on the surface of the gate electrode layer 16, and also according to whether or not the target biomolecules bind with the immobilized probe biomolecules 18. Thereby, the target biomolecules can be detected by measuring the variance in the intensity of the current flowing through the channel. The conventional FETs as described above have a structure such that the probe biomolecules 18 are immobilized on a surface of the channel.
In conventional FETs, the current flowing through the channel of the FET is greatly affected by the ion concentration of an analyte, and sensitivity of the conventional FET is poor. The reason for the poor sensitivity of the FET is that an increase in ion concentration of the analyte causes masking of molecular charges and subsequent deterioration of the sensitivity of the FET for use as a sensor. U.S. Patent Application No. 2006001191 (“the 191 patent application”) discloses a method of analyzing a polymerase chain reaction (“PCR”) product by purifying the PCR product with a buffer, such as a 10 millimolar (mM) tris hydrochloride (“Tris-HCl”) buffer, spotting the purified PCR product on a FET to which polylysine is immobilized, washing and electrically detecting the PCR product in a 0.01 mM potassium chloride (“KCl”) solution. Thus, the method of the '191 patent application necessarily includes the process of washing. However, in the case of a lab-on-a-chip (“LOC”), the LOC is required to perform a series of processes on the same chip, including measurement, as well as collection, separation, amplification and purification of the sample to be analyzed, and solutions used in these serial processes typically have high ion concentrations.